This invention relates to a hydraulic damper adapted for use in a suspension system of a vehicle and, particularly to the hydraulic damper of the kind including a generally vertical cylinder receiving therein hydraulic liquid, a piston working in the cylinder and partitioning the interior thereof into two liquid chambers, one or a pair of damping force generating valves mounted on the piston for generating the damping force both in the extension and contraction strokes of the damper, a piston rod secured to the piston and extending through the upper end of the cylinder to the outside, a coaxial bore and a radial hole formed in the piston rod to constitute a by-pass passage connecting the two liquid chambers, an adjustable valve member for steppingly controlling the effective area of the by-pass passage, and an actuating rod extending through the coaxial bore in the piston rod with the inner end thereof being secured to the valve member.
The hydraulic damper of the aforementioned kind has recently proposed so as to adjust the characteristics of the damper to match with the driving condition of the vehicle such as the running speed of the vehicle and the condition of the road on which the vehicle is running. When the vehicle is running on a smooth road, the driving comfortability can be improved by decreasing the damping force of the damper and, when the vehicle is running on a rough road, it is possible to prevent excessive vertical movement or vibrations of the vehicle by utilizing stiff hydraulic dampers or by increasing the damping force of the damper. Further, it is experienced that when the vehicle is running at a high speed it is preferable to increase the damping force of the damper, otherwise, the direction of the vehicle cannot stably be maintained. The adjustable valve member can adjust the damping force by at least two distinct steps, so that the damper can be adjusted betwen the high, low and, if desired, the medium damping force conditions. The adjustment of the adjustable valve member is usually performed by rotating or axially displacing the actuating rod.
For operating the actuating rod, an electric motor or electromagnetic solenoids have been utilized. However, there are shortcomings such that, when an electric motor is utilized to operate the actuating rod, the electric motor should be accompanied with a complicated electric circuit including such as potentio-meters and the like for stopping the motor at precisely predetermined positions, and that when electromagnetic solenoids are utilized to actuate the actuating rod, the number of solenoids should correspond to the number of adjusting steps. In short, according to prior art, it is difficult to steppingly and reliably adjust the adjusting valve with a simple constitution.